Obviously, at point A , the satellite is farther from the center of rotation than at point B . Conservation of
angular momentum tells us that, correspondingly, the angular speed at point A must be less than at point B .
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The other really famous example of conservation of angular momentum involves a spinning figure
skater. When a skater spinning with his or her arms outstretched suddenly brings the arms in close to the
body, the speed of rotation dramatically increases. Rotational inertia decreased, so angular speed
increased.
You can demonstrate this phenomenon yourself! Sit in a desk chair that spins, and with your legs
outstretched, push off forcefully and start spinning. Then tuck in your feet. Dizzying, isn’t it?
Practice Problems
Multiple Choice:
1 . All of the objects mentioned in the choices below have the same total mass and length. Which has the
greatest rotational inertia about its midpoint?
(A) a very light rod with heavy balls attached at either end
(B) a uniform rod
(C) a nonuniform rod, with the linear density increasing from one end to the other
(D) a nonuniform rod, with the linear density increasing from the middle to the ends
(E) a very light rod with heavy balls attached near the midpoint2 . A pool ball is struck at one end of the table; it moves at constant speed to the far end of the table. A
camera is mounted at the side pocket at the table’s midpoint, as shown. From the camera’s point of
view, the pool ball travels from right to left. At which point in its motion does the ball have the
greatest angular momentum about the camera’s position?
(A) when the ball was first struck
(B) at the midpoint of the table
(C) the angular momentum is the same throughout the motion
(D) at the far end of the table
(E) one-quarter of the way across the table, and then again three-quarters of the way across the table